Electrophotography is an image forming method wherein a photoreceptor is electrostatically charged with and imagewise exposed to light to form an electrostatic latent image, the electrostatic latent image is then developed with a developer, and the resulting toner image is transferred onto a transfer paper and fixed to obtain an image. The photoreceptor for use in electrophotography basically comprises a photoconductive layer formed on a conductive substrate. Amorphous silicon (hydrogenated amorphous silicon) has been used as the material for the photoconductive layer in recent years, and many improvements have been attempted. Amorphous silicon photoreceptors using amorphous silicon are prepared by forming an amorphous layer of silicon on a conductive substrate, for example, by discharge decomposition of silane (SiH.sub.4) gas. Hydrogen atom is introduced into the amorphous silicon layer to thereby impart good photoconductivity. The amorphous silicon photoreceptors have such characteristics that the photosensitive layer has a high surface hardness, excellent wear resistance, excellent heat resistance, excellent electrical stability, a wide range of spectral sensitivity, and high photosensitivity. Accordingly, the amorphous silicon photoreceptors have suitable properties as the electrophotographic photoreceptors as described above.
However, the amorphous silicon photoreceptors have a disadvantage in that dark resistance is relatively low, and hence the dark attenuation of the photoconductive layer is large, and a sufficient charging potential can not be obtained when the photoreceptors are charged, though the amorphous silicon photoreceptors have excellent characteristics as the photoreceptors. Namely, the amorphous silicon photoreceptors have a disadvantage in that when the amorphous silicon photoreceptors are charged and imagewise exposed to light to form an electrostatic latent image and the electrostatic latent image is developed, surface charges on the photoreceptors are attenuated until imagewise exposure to light, or charges in the unexposed area are attenuated until development, and hence a charging potential required for development can hardly be obtained.
The attenuation of the charging potential is apt to be affected by environmental conditions, and the charging potential is greatly lowered particularly under high temperature and humidity conditions. Further, when the photoreceptors are repeatedly used, the charging potential is gradually lowered. When the electrophotographic photoreceptors that exhibit the large dark attenuation of the charging potential are used to obtain images, the image density becomes low and the reproducibility of half tone becomes poor.
Attempts have been made in which a surface layer of amorphous silicon carbide, amorphous silicon nitride, or amorphous silicon oxide is formed on the photoconductive layer comprising amorphous silicon by plasma CVD process to improve the above-described disadvantage.
However, when the amorphous silicon photoreceptors having such a surface layer as described above are repeatedly used to obtain images, faint images occur. This phenomenon is remarkable particularly under high humidity conditions, and such photoreceptors can not be used in practical electrophotographic processes.
Further, the amorphous silicon layers prepared by plasma CVD process have disadvantages in that the amorphous silicon layers are apt to the cracked and have poor impact resistance in comparison with selenium photoconductive layers and organic photoconductive layers, though the amorphous silicon layers have a high surface hardness. Accordingly, the photoreceptors mainly composed of amorphous silicon are liable to be marred by paper releasing grippers, etc. in duplicators or printers. As a result, white spots or black spots are liable to be formed in resulting images.
Furthermore, the amorphous silicon photoreceptors have many defects having a semispherical form of 1 to 80 .mu.m in diameter on the surface of the photoconductive layer, and when image formation is repeatedly conducted, electrical and mechanical breakage occurs in the defected parts of the layer, and white spots and black spots appear on the image by breakage of the layer, whereby the image quality is damaged.
The present inventors have made studies and found that when the photoreceptors have the amorphous layer mainly composed of silicon, nitrogen or carbon formed by the plasma CVD process on the surface thereof, the photoreceptors are thermally and mechanically stable, and further they are photoelectrically stable in the electrophotographic process, but they are unstable against oxidation in comparison with other materials, and oxide films formed on the surface thereof are more active than layers of organic and inorganic high molecular weight materials against moisture and the adsorption of corotron products. The present inventors have found that breakage of the defected parts of the layer can be prevented not by concentrating an ion stream from corotron into the flaw parts of the layer, but by dispersing the ion stream from corotron without concentrating an ion stream into the defected parts, which breakage has been conventionally considered to be a factor by which the life of the amorphous silicon photoreceptors is limited. The present inventors have previously proposed an electrophotographic photoreceptor having a surface layer of an organic or inorganic high molecular weight material containing fine particles of a conductive oxide dispersed therein, as described in JP-A-4-88350 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, the present inventors have found that the aforesaid electrophotographic photoreceptor has still a disadvantage in that when an image formation process is repeatedly conducted to obtain many copies of as much as 300,000 copies or more, faint images occur, and the photoreceptor is marred by paper releasing finger made of iron. The present invention is intended to overcome the above-noted problems associated with the prior art.